There is a growing trend in the aerospace industry to expand the use of composite materials for a diverse array of structural and dynamic applications. One particular application for the use of composite materials lies in the fabrication of main rotor blades for helicopters.
With increased usage of composite materials to fabricate main rotor blades, the helicopter industry is continually seeking to improve the tooling and/or methods used to fabricate main rotor blades so as to reduce the per unit fabrication costs associated with the main rotor blades. Typically, the per blade fabrication costs are higher than need be due to part rejections or rework that occurs during the main rotor blade fabrication process. Part rejections typically occur where the composite material has been so substantially damaged during the fabrication process that rework is not cost effective or where a finished fabricated part exceeds the tolerance limits established for the part. Rework occurs where the composite material has been damaged during the fabrication process, and the damage may be repaired in a relatively cost effective manner.
Sikorsky Aircraft has developed a parallel manufacturing protocol for fabricating helicopter main rotor blades wherein a blade subassembly and a leading-edge sheath are concurrently fabricated as individual components, and then the prefabricated blade subassembly and the prefabricated leading-edge sheath are integrated in combination to form an assembled main rotor blade. The assembled main rotor blade is subsequently cured to form a finished main rotor blade. This protocol was adopted in large measure because experience has shown that the leading edges of main rotor blades are subjected to varying degrees of abrasion during helicopter operations. As a result of such abrasion effects, the leading edge of a helicopter main rotor blade at some point becomes aerodynamically unsuitable for further use. Rather than replacing the entire main rotor blade, it was determined that a replaceable leading-edge sheath would allow abrasion-degraded main rotor blades to be efficaciously and economically repaired.
The prior art process for fabricating blade subassemblies involved the use of a "clamshell" tooling fixture and a "wet" lay-up process for the composite materials. It was determined that the rejection rate for blade subassemblies fabricated using the clamshell tooling fixture and the wet lay-up process was unacceptable in light of the today's competitive market. The dependability and accuracy of the clamshell tooling fixture depended upon the stability of the laid up tooling contours, the proper securing and pinning of all fasteners and locators, and the variability in applying blade outer mold line pressures. The clamshell tooling fixture and the wet lay-up process were subjected to shrinkage and lose of tolerances, which led to component rejection. The clamshell configuration result in asymmetrical pressure distributions across the layed-up blade subassembly.
Another area of concern in the parallel manufacturing protocol was the sheath spreader tool used to integrate the leading-edge sheath in combination with the blade subassembly. The leading-edge sheath has a prefabricated configuration that does not allow the sheath to be inserted directly onto the blade subassembly. Rather, the aft edges of the leading-edge sheath must be spread apart to allow for leading-edge sheath to be inserted onto the blade subassembly. The prior art sheath spreader tool comprises segmented angular stainless steel sheet metal grabbers that are mounted spanwise on the aft edges of the leading-edge sheath in contact with the inner mold line (IML) surfaces (which are formed of composite material) of the leading-edge sheath. Each segment of the prior art grabber is individually actuated by means of a side cam lever. The prior grabbers exert a shearing action against the IML surfaces of the leading-edge sheath in spreading the aft edges of the sheath apart. The shearing action caused by the prior art grabbers caused cracks and delaminations in the composite material subjected to the shearing action thereof, resulting in component rejections or rework. In addition to the foregoing deficiency of the prior art leading-edge sheath spreader tool, the segments of the grabber are individually actuated in a sequential manner such that to spread apart the entire leading-edge sheath involves multiple, repetitive operations. Not only is such a procedure labor intensive and time consuming, and hence costly, such a procedure may induce unwanted stresses into the aft edges of the leading-edge sheath.
A need exists to provide an apparatus for spreading a leading-edge sheath for insertion onto a blade subassembly without inducing cracks and/or delaminations in the composite material of the leading-edge sheath. Preferably, the apparatus should spread the leading-edge sheath apart in a single operation to reduce the time required to spread the leading-edge sheath apart. A need also exists to provide a fixture for assemblage and compacting of a blade subassembly that provides a uniform pressure distribution during the compaction of the blade subassembly, that facilitates the use of prepreg composite materials, and that ensures proper chordwise and spanwise alignment of the components of the blade subassembly layed-up in the fixture. A need also exists to provide a sheath spreading apparatus and compaction fixture which in combination simplify the insertion of a spread-apart leading-edge sheath onto the blade subassembly.